A major worldwide problem is the disposal of waste. Typically, wastewater is treated to separate the water, which is reclaimed, from the solids which are disposed of, often by drying and burning. In the United States of America, about 25% of municipal sludge is disposed of by incineration.
Waste sludges produced from wastewater treatment are typically dewatered to 16 to 25% total solids with belt presses or centrifuges and 26-35% with filter presses. The resulting wet cake is then dried and burned to yield ash and gaseous products which must also be completely oxidized (burned) and cleaned.
While sewage sludge has a heat content similar to a low ranked coal or lignite, the bulk of its thermal energy is derived from distilling the volatile content to gases (vapors) which are then combusted. (This is much different from higher order coal where the majority of the energy is stored as fixed carbon.) The distilled gases, such as methane and ethane, will be combusted to CO.sub.2 and H.sub.2 O when there is adequate available oxygen and the ignition temperature is maintained with sufficient detention time. The required ignition temperature can exceed 1400.degree. F. and contact up to 3 seconds may be required. Even at ignition temperature, if there is insufficient oxygen, partial combustion will occur which can yield carbon monoxide, soot (carbon) and gaseous hydrocarbons. Additional contaminants include various volatile components distilled from the drying sludges the quantity of which varies in response to changes of locations of the drying and combustion hearths. As a result of these inputs, stack gases analyzed by EPA test procedures may identify the following contaminants:
Hydrocarbons--small molecular gases such as methane and ethane.
Products of Incomplete Combustion--intermediate compounds such as acrylonitrile, benzene, and vinyl chloride.
Distillates--larger volatile compounds which are water soluble and are trapped by the back-catch in the test procedure such as alcohols, acetates, and condensible hydrocarbons, as well as more complex organics.
Particulates--the `dust` loading in the stack which is not removed by the scrubbers. Particulates (soot) increase when partial combustion occurs.
Air pollution standards are becoming more restrictive and in some areas non-attainment of existing standards can pose serious problems even for the highest quality exhaust gases. Tighter standards must be anticipated in the implementation of any changes to improve performance of furnaces, many of which are already having difficulty meeting existing standards. It has often been necessary to raise the temperature of gases to 1400.degree. F.-1500.degree. F. and reduce the sludge loading by 30% to meet air pollution standards. Some states have already imposed minimum exhaust temperatures (after last contact with sludge) of 1200.degree. F. to 1400.degree. F.; and federal and state laws under consideration may require a minimum afterburner gas temperature as high as 1700.degree. F.
In prior sludge disposal practice, the off-gases from the cake drying zone of the furnace contain entrained particulates, are usually wet due to a content of large quantities of water vapor, and contain undesirable volatile impurities. Before the gases can be discharged to atmosphere the particulates and other contaminants must be removed.
The only known effective method of eliminating organic and other combustibles from off-gases in high temperature retention in the presence of adequate oxygen. A retention time of 1 to 3 seconds and a temperature in the range of 1400.degree. F.-1700.degree. F. are normal requirements with the shorter detention time corresponding to higher temperatures. Exact operating conditions will depend on the specific requirements, waste constituents and/or regulatory statutes.
In accordance with prior practice, off-gases derived from drying and burning sludge in a counter-flow furnace are deodorized and cleaned by subjecting them to combustion in a so-called afterburner wherein the combustible gas content, augmented as needed by auxiliary fuel and combustion air, is burned to heat the gases sufficiently to effect combustion of the contaminants. To meet emission standards, this often requires heating the gases to 1400.degree. F.-1500.degree. F. or higher. Thereafter, the gases are scrubbed and otherwise treated to remove particulates and moisture and to cool them for eventual release to the atmosphere.
In some cases, the afterburner is established in the furnace itself adjacent the gas-outlet sludge-inlet zone, (the top hearth of a multiple hearth furnace), while in other cases a completely separate unit is utilized. In either case, the dirty, wet off-gases, which are already at a temperature of at least 500.degree. F., must be further heated sufficiently to ignite and burn their combustible content. This requires heating the off-gases to 1400.degree. F. to 1500.degree. F. and takes considerable added fuel because it is necessary to heat not only the contaminants to be burned, but also the moisture, fuel combustion air and the excess air used in the furnace. Excess air is stated as a percentage of stoichiometric air. Stoichiometric air is the amount required to fully oxidize organic solids containing carbon, hydrogen, oxygen, nitrogen and sulfur in the sludge and fuel supplied to the furnace. Excess air is commonly equal to 100% to 125% of stoichiometric air.
Although the existing systems have operated for many years, most of them will require modifications to enable continued operation under more restrictive air pollution codes. As noted, the only known method of meeting air pollution control criteria is high temperature combustion. High temperature combustion is expensive. It requires a lot of fuel and suffers from the inherent disadvantage that the additional fuel uses up a lot of air normally used in sludge combustion thereby reducing the furnace's sludge handling capacity, sometimes by as much as 30%-40%. The capacity loss can be overcome by extensive modifications to the furnace air supply and scrubbing systems, but the high operating expense of extra fuel will remain as will the expense of supplying and handling greater volumes of air and resulting off-gases.
A major cost factor of prior systems is afterburning of the off-gases containing the products of sludge drying and combustion. These gases, which are wet and loaded with particulates and other contaminants, cannot be used in heat exchangers because they would quickly foul the equipment and create a fire hazard. Because of this, the off-gases are subjected to afterburning as they are (wet and dirty) then immediately scrubbed and cooled; and none of the heat content is available to preheat the off-gases.
A related problem inherent in existing systems from the need to prepare (heat) the off-gases for immediate afterburning. The economics of afterburning often require that the furnace temperature be so high as to result in undesired `fuming` of heavy metals in the furnace with consequent difficult problems of heavy metal removal.
In summary then, the wet, dirty off-gases from the furnace contain moisture, particulates, condensibles and extra gas volume which require considerable fuel to reach combustion temperatures in the afterburner, but which make no contribution to the cleaning or deodorizing operation.
From the foregoing, it is evident that in the current systems of sludge disposal, the wet, dirty, medium-temperature off-gases are the real culprits working against improvement in the system. This invention addresses the problem by conducting the sludge cake drying and burning in the furnace in a manner to yield dirty off-gases which are then handled in a unique manner before they are subjected to increased oxidative temperatures in the afterburner.